We are using the rat as a model to study the bio synthesis and metabolism of plasma applipoprotein B (apoB), the structural protein of very low density lipoproteins (VLDL), and apoA-I, the major protein of high density lipoproteins (HDL). An electrophoretic method was developed that separates three forms of apoB with different apparent molecular weights (B-100, B-95, and B-48). Isolated livers perfused with blood containing [3H]leucine produce all three forms of apoB, but not in constant proportions. Studies with rats on different diet suggest that liver produces multiple types of VLDL different diet suggest that liver produces multiple types of VLDL differing in their apoB component. With increasing hepatic triacylglycerol production, more VLDL with B-48 is produced. Furthermore, VLDL with B-48 is cleared more rapidly from the circulation. Together, these results suggest the outlines of a mechanism that may help to control the blood triacylglycerol concentration and to determine the metabolic fate of hepatic VLDL on the basis of its apoB component, which is physiologically regulated in the liver. Rat plasma contains two main forms of apoA-I differing in net charges and separable by two-dimensional gel electrophoresis. About 70% of plasma ApoA-I is the more basic isoform. Only liver and small intestine produce apoA-I. More than 85% of the apoA-I released by isolated, perfused rat livers and also by isolated small intestine is the more acidic isoform. Only liver and small intestine produce apoA-I released by isolated, perfused rat livers and also by isolated small intestine is the more acidic isoform. Also, the nascent protein recovered from both tissues after pulse-labeling with [3H]leucine in vivo or in vitro is the acidic isoform. After its release into the blood in vivo, the acidic isoform is gradually converted to the more basic form, explaining the steady-state isoform pattern. The conversion is sluggish and incomplete in vitro, suggesting a labile converting factor. Conversion requires a divalent metal. Preliminary evidence indicates the acidic form released by liver and intestine is a proapoA-I from which an N-terminal hexapeptide is cleaved in the circulation. A similar conversion of proapoA-I to apoA-I was recently reported to occur in man. The significance of this unusual intravascular conversion remains unknown but its study opens a new approach for investigating the functions of apoA-I.